Process of bleaching wood pulps and an apparatus for carrying out said process



Aug. 7, 1934. D. B. DAVIES PROCESS OF BLEACHING W0 OD PULPS AND ANAPPARATUS FOR CARRYING OUT D PROCESS Filed.April 2 Sheets-Sheet l\\\\\\\\\\\\Iil!I///////// g r .INVENTORV pam'd 3, p4 wa.

B W W ATTORNEY Aug. 7, 1934. D. B. DAVIES 1,968,994

PROCESS OF BLEACHING WOOD PULPS AND AN APPARATUS FOR CARRYING OUT SAIDPROCESS Filed.April 8, 1931 2 Sheets-Sheet 2 IN VENT OR A TTORNEYPatented Aug. 7', i934 PROCESS OF BLEACIHNG WOOD PULPS Am AN APPARATUSFOR CARRYING GUT SAID PROCESS David B. Bavies, Shelton, Wash.Application April 8, 193i, Serial No. 528,514

2 Claims.

My invention relates to the process of bleaching wood pulps and to anapparatus for carrying out said process. More particularly, myinvention. relates to a process characterized in 5 having a circulationof pulp whereby one part of the pulp moves in an upward direction withrespect to the remainder of the pulp without confining said parts toseparate chambers; and

to an apparatus or bleacher cell having (a) w levelling-arm'sfunctioning to determine when the desired charge of material is suppliedto the apparatus and to circulate the material, said arms preferablyhaving means to conduct bleaching solution and sprinkle the sameuniiormly over the top of the charge of material;

(b) means veyor member of the apparatus to remove material from saidmember; andic) being without chamber forming casing or housing.

The bleaching process of, and the apparatus for bleaching of, wood pulppresents a complex problem, and any successful process or successfulapparatus for carrying out the process must be characterized by beingcapable of variation to meet the varying character of the pulp, which inturn is varied according to the purpose for which the pulp is to beused. The bleaching action, it will be understood, involves,particularly in the initial stages, purification of the wood pulp and isnot confined to changing the color to white.

Wood pulp has the property of acting as a blotter and readily absorbsliquid to which it may be exposed, or with which it may come in contact.Therefore, the process and appara: tus must be such as to thoroughlyexpose every portion of the wood pulp to direct contact with thebleaching solution introduced. This in turn means that the circulationof the wood pulp must be most thorough-no dead pockets must bepermitted. It will be understood that any portion that is not bleachedmay operate to discolor the portion which is bleached upon the chargebeing removed from the bleacher. Bleaching being in part a purificationprocess, uniformity of action helps form alpha, beta and gamma cellulosein proper proportions, all of which seem to be desirably present anddetermine the properties of the final product, whether in the form orpaper or rayon.

Wood pulp is further characterized by the property of being readilydiscolored by the presence of foreign material, and particularly is thistrue of iron, either in the form of small metallic particles which maybreak loose from to direct water against the conparts of the apparatusconstructed of iron after corrosion. due to the action of the chemicalsemployed in treating the pulp, or to the iron which may be dissolved byreason of the chemical agents employed to treat the pulp acting uponparts of the apparatus constructed of iron, or to iron in colloidalform. Therefore, it is particularly advantageous, if not positivelyessential, to eliminate all the internal parts of the machine which areconstructed of iron or other metals. Wood pulp must be free as possibleof dirt particles or specks. In fact, pulp is graded according to thedirt count. The apparatus in common use constitutes an actual source ofcontamination.

An object of my invention is to eliminate from the bleaching apparatusall parts of metal construction as far as possible, since the iron orother metals are subject to being chemically acted upon by thebleaching, i.e., chlorine, solution, thus constituting a source of dirtof iron particle nature and iron in solution, or iron in colloidal form.

Thus iron or other metal parts of the apparatus constitute a source ofmetallic particles and iron in solution or colloidal form, all of whichoperate in the final process to discolor andotherwise degrade the pulp.The particles of iron or other metals may not at first discolor thepulp, but such particles, when the particular pulp is acted upon bychemicals employed to produce rayon, may discolor the rayon. That is,

the iron particles in subsequent treatment may become highlyobjectionable in the form of dissolved iron. Heretofore the commonstandard practice bleacher generally employed embodies a screw conveyorwhich operates in an axially disposed chamber formed by a me'illiccasing in the bleaching chamber, said casing directly surrounding saidscrew conveyor. Heretoiore it has always been deemed necessary to thusprovide two chambers within the bleacher, the inner one in which thescrew conveyor is disposed, and the outer, annular chamber. In thisstructure commonly employed and in. general use, the material in theinner chamber is thus positively separated from the material in theouter surrounding chamber.

A primary object of my invention is to provide an apparatus or bleachercell which does not have this axially disposed casing; and it issimilarly my object to provide a process characterized by its thoroughcirculation of wood pulp wherein that part of the pulp in the inner oraxial region is moving upwardly by aid of 'termined is the mined.Thereupon,

section is moving downwardly while the parts of said oppositely directedmoving parts of pulp are in direct intimate contact with each other.

A further problem in providing a bleacher is to provide one in which nopart of the pulp can be subjected to a rebleaching process after saidpulp has been once fully bleached.

Serious objection obtains to the common practice form of bleacher havingthe casing forming an inner chamber because at the conclusion of thebleaching process there remains in the axially disposed chamber arelative large proportion of the charge, which is fully bleached, saidportion in said chamber amounting to as much as one hundred cubic feet.It is very difficult to remove this portion of the charge within theaxially disposed chamber and ordinarily this portion is allowed toremain and become rebleached with the next charge. To enable theconveyor to lift the material in the inner chamber requires the pressureof constantly newly fed material at the bottom and the consistency ofthe pulp is such that running the conveyor backwards fails to eliminateit. The rebleaching forms undesirable compounds which seriously affect,by direct action or by acting catalytically, the value of the charge. Anobject of my invention is to eliminate this difliculty and provide ableaching apparatus from which the entire charge may be removed and noportion subjected to being rebleached. Again, owing to the highabsorption property of wood pulp, it is important that the bleachingsolution should be put into direct contact with every part of the pulp.Pulp should not be bleached indirectly, that is, it should not ingsolution through contact with other pulp containing the bleachingsolution, but rather, so far as possible, every part should itself bebrought into direct contact with the bleaching solution. There should beno spotty high bleach solution concentration sections in the charge butthe solution should be evenly distributed throughout.

It is the object of my invention to distribute the bleaching solutioncharge, and to this end vided for distributing, immediately upon itsintroduction, uniformly the bleaching solution over the top of the woodpulp, that is, over the ascending as well as the descending column ofpulp. The introduction of the solution according to my invention is suchthat it can. be done even while the pulp is being introduced.

A further object of my invention is to provide uniformity in volume ofcharges to the bleaching apparatus. The importance of this is thusexplained. It is very important toknow and regulate the exactrelationship between the bleaching solution and the quantity of pulp. Indoing this the first step is' to know the consistency or density of thepulp, that is, the percentage of water present. The next fact that mustbe devolume of the pulp present, that is, the charge volume. Thus, it isimportant to know when you have a full charge so that this charge volumefactor is relatively constant. Pulp, due to its varying character, formsa cone with bleacher cell of varying angles; therefore, it is importantto have some means of knowing when; the cell is fully charged asrespects volume. The percentage of water is :separately deterwith thesefacts ascertained the percentage of bleaching solution to be added toevery portion of the special means are proonly receive its bleach-- -acovering 8 for the top. A

.7 1,968,994 the conveyor, and that part in the peripheralis correlated.For this purpose it will be under stood that while the cell is beingcharged a portion, as a sample, of the material is removed while enroute to the cell and placed in a centrifuge machine; then thecentrifuge delivers a sample of a definitely known moisture content. Toknow precisely the volume of the pulp present in the charge isimportant, then in order to utilize the proportions revealed bytreatment of the sample, and to know the charge volume present, allfluctuations incident to varying cones must be eliminated.

It will be understood that the character of the pulp supplied to thecell varies and this variation is intentional and is determined by theparticular purposes for which the pulp is to be used. This means varyingquantities of the bleaching solution must be applied.

Having the water content determined by testing said sample, a well knownpermanganate of potash test is next applied to determine what is knownin, the trade as the bleachability of the pulp.

Having thus determined the bleachability and the volume of the standardcharge, it is then known what quantity -of the bleaching solution isrequired to get the degree of the bleach desired for the particularproduct for which the particular charge is to be used. Thus, uniformityin volume of charges is a fundamental Fig. 3 is a view in section of amodified form of the bottom portion of the same;

Fig. 4 is a view of a further modified form of the bottom portion ofsame with plow;

Fig. 5 is a plan view of the levelling-arms;

Fig. 6 is a view of levelling-arms of modified form, i.e., pipeconstruction;

Fig. 7 is a view of levelling-arms of modified form, i.e., boxlikeconstruction; and

Fig. 8 is a view in section on line 8-8 of Fig. 7.

The bleacher or bleacher cell, ordinarily running to twelve feet indiameter and fourteen to eighteen feet high, is formed with a verticallydisposed chamber 6 having walls 7 of suitable material, preferablyconcrete and tile lined, and shaft 9 is mounted in the chamber 6 havingactuating gears 10 and 11 operatively connected to a source of power. Ascrew or conveyor flight 12 is keyed upon the shaft 9 so as to turn withsaid shaft and terminates at about the height or level to which thecharge is desired to fill the chamber.

Radially disposed levelling-arms 13, which may be of suitableconstruction as V-shaped (cross-sectionally considered) trough-likemembers, or may be of pipe construction or box-like in cross-sectionwith solution conducting pipe therein, are keyed to the shaft 9 so as toturn therewith and cause the pulp material, which is of a consistencywhich forms or stands as a cone vertical, longitudinal upon being fedinto the chamber, to lie substantially fiat across most of the top ofthe chamber at the charging level 14, indicated by the dotted line.These arms 13 preferably have perforations 15 from which the bleachingsolution may be uniformly spread over the charge of pulp, including theperiod of charging the cell. These t arms 13 are supplied by bleachingsolution running down through perforations 16 in an annular receptacle17 mounted on arms 13. A conduit 18 conveys the bleaching solution tothe receptacle 17 and is in turn itself connected to a suitable sourceof supply of said solution.

A conveyor screw 19 in a trough supplies the pulp material to thechamber 6, through trough opening 21, which may be closed by slide 22.Conduit 18 is led in through opening 21- An opening 23 in the covering 8may admit a nozzle 24 disposed to be turned through varying angles topermit directing a stream of wash water against all portions of theconveyor flight to remove all pulp which has been bleached in a givencharge and thereby preventrebleaching, which due to the chemical actioninvolved is very deleterious to the product as a whole. This nozzle 24may be secured in the opening in a flexible member 25 and adapted to betrained in varying angles, or it may have no direct connection to thecell structure but inserted only when the cell is to be discharged.

The bottom of the chamber 6 is preferably shaped so as to cause the pulpto positively flow to the section about the screw 12 where it will beacted upon by the screw and elevated. The convex curve 26 will aid andpositively insure the pulp while being held down by the super-imposedpart of the charge, to flow to the elevating conveyor flight 12. Thebottom may have the form shown in Fig. 3. The curved forms of the bottomconstitute means in the bottom of the chamber to cause the material toflow to the conveyor. Again, it may have the flatter curved bottom shownin Fig. 4, but here it will require the plow 2'! to. draw the pulp tothe screw 12 and eliminate pockets in the material, in which pockets thematerial would fail to be properly circulated and not being properlycirculated would fail to be properly bleached.

In the bottom, outlet port 28 is closed by sliding gate 29 operativelyconnected to rod 30, the turning of which causes gate 29 to move in andout. Hand wheel 31, rod 32 and gears 33 and 34 enable the gate to beoperated from the top of the bleaching cell. Inlet pipe 35 for washwater permits a complete washing of the bottom of the chamber 6. Thelevelling means may be arms 36 (Fig. 6) of pipe construction, or theymay be arms 38 (Fig. 7) of box-like construction with a pipe 39 mountedinthe box-like housing in communication with the annular receptacle 1'7.Small branch pipes 40 bend backward from the direction of rotation, sothat pulp does not fall into the arm and interfere with the flow of thesolution or the small parts become clogged.

The mode of operation of much of the device has already been set forthabove, and will be readily understood from the drawings. It will benoted that there is no casing about the screw or conveyor flight 12. Ihave discovered that the material is of such consistency that it willtravel up the conveyor and be thoroughly circulated without anycentrally disposed casing and hence all injurious effect of iron inparticle form or in solution, or in colloid form, is obviated. Thisprovides a product peculiarly suited for the manufacture of paper,having a high degree of whiteness.

Such casing to form the two concentric chambers has been deemednecessary heretofore to insure proper circulation of the pulp to providethoroughness, uniformity, and efficiency of the bleaching operation. Byconstruction of a bleacher of standard proportions and operation inconjunction with all the remaining units of a pulp mill, I havediscovered and proven that the cell does not need to have the casing toform the two chambers, and have thus provided for the elimination of thesource of contamina-.

tion of the pulp as herein explained. Obviously the bleaching unit mustoperate efficiently and in conjunction with the remaining units of theplant or it would be wholly impracticable. The levelling-arms functionalso to stir up the pulp and move it to the outer wall.

In treating the highly complex substance wood pulp, the character(chemically and physically) of the material presents special problems.High concentration of the bleaching solution may form deleteriouscompounds. Pulp, being of a fibrous character and being composed of avariety of compounds some of which are sticky, is liable to become lumpyand, but for the levelling-arms which break up, pull and push thematerial into other sections of the chamber in the act of spreading, itcould not be readily formed into a homogenous mass with the bleachingsolution distributed uniformly therethrough. The pulp is forced upwardlyaxially against the leveling-arms and they function to move the materialpromptly to the peripheral portion .of the chamber and, in thus promptlyremoving it from the center, assist in providing efiicient circulation.Moreover, not only must the mass be made finally homogeneous but, at alltimes during the period of treating the charge, high concentration ofthe solution in limited sections should be avoided and uniformdistribution of the bleaching solution is accomplished in my inventionfrom the very moment the material enters the chamber as thelevelling-arms spread the solution throughout the diametrical area ofthe chamber. This even distribution of the bleaching solution is trueeven when the material is being fed into the device, i. e., even whenthe chamber is only partly filled. The weight of the material in theportion of the chamber peripherally disposed with respect to theconveyor fight 12 assists the conveyor to elevate the axially disposedportion of the pulp and the moving of the two portions in oppositedirections and in contact with each other assists in providingthoroughness in intermixing of the pulp and the bleaching solution.Hence, the omission of the usual casing about the elevating screw ishelpful in other ways than eliminating a source of contamination; Thus,in many ways the device embodying the various features of my inventionis characterized by a mode of operation differing from that of thedevices commonly in use.

I claim:

1. The process of bleaching wood pulp comprising the steps of isolatinga batch of said wood pulp characterized by its plastic-like and itsmoist but apparent liquid-free inter-locking fibrous nature; causing acirculation of said material within itself by passing a portion of saidmaterial in an upwardly directed relatively small and substantially welldefined stream through a relatively greater body of the same materialmoving downwardly at a relatively much slower speed than said streammoves upwardly, said stream throughout its length be-' ing in directcontact with said body of material, whereby a considerable frictionbetween, and a rubbing and kneading action of, the contacting portionsof the ascending and descending material, is developed and providedrespectively; subjecting said material, while in the small stream, to asqueezing or pressing action to partially eliminate the spent orrelatively inactive bleaching solution, together 'with some of thediscoloring incrusting material; releasing said material from the saidpressing and squeezing action to open the mass of fibers to permit theblotting action which characterizes said pulp to take place andreexposing the material to active bleaching solution; and then passingsaid exposed material to the descending body of the material to completeand repeat the circuit whereby uiformity of bleaching the wood pulpthroughout is provided.

2. The process of bleaching wood pulp comprising the steps of isolatinga batch of said wood pulp characterized by its plastic-like and itsmoist but apparent liquid-free interlocking fibrousnature; causing acirculation of said material within itself by passing a portion of saidmaterial in an upwardly directed relatively and a rubbing and kneadingaction of, the contacting portions of the ascending and descendingmaterial, is developed and provided respectively; subjecting saidmaterial, while in the small stream, to a squeezing or pressing actionto partially eliminate the spent or relatively inactive bleachingsolution,- together with some of the discoloring incrusting material;releasing said.

material .from the said pressing and squeezing action to open the massof fibers to permit the blotting action which characterizes said pulp totake place and reexposing the material to active bleaching solution;subjecting said material after the squeezing pressure is released to araking action to forcibly break up the somewhat matted or moldedmaterial and expose the individual fibers to the active bleachingmaterial; and then passing said exposed material to the descending bodyof the material to complete and repeat the circuit, whereby uniformityof bleaching the wood pulp throughout is provided.

DAVID B. DAVIES.

